We have determined the 1.9 E crystal structure of the isolated, trimeric, catalytic (C) subunit of the allosteric enzyme, aspartate transcarbamoylase (ATCase). Surprisingly, the active C trimer resembles the relatively inactive, unliganded, T-state holoenzyme, except that the trimer lacks three-fold symmetry and the active sites are partially disordered. The flexibility of the C trimer, contrasted to the highly constrained T-state holoenzyme, reveals that regulation of ATCase involves modulating the potential for conformational changes essential for catalysis. Large differences between the active C trimer and the holoenzyme: bisubstrate-inhibitor complex call into question the generally accepted view that this complex represents the activated R-state of ATCase.